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THE BASIC-BESSEMER PEOCESS.                            115
phosphate of iron; but this would immediately come in contact with a silicious slag, or, in other words, with a slag possessing more than enough silica to meet the requirements of its bases, and the
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silica being immediately seized by the oxide of iron, the unprotected phosphoric acid would be robbed of its oxygen by the metallic iron. This may seem a long explanation of the simple fact that phosphorus does not oxidize, but there are reasons for supposing that in many chemical actions the atoms are in a state of general translation, so that while many compounds are formed, only those remain which find a suitable environment. It is difficult to explain the formation of phosphoric acid in the basic converter without assuming an action which can just as readily obtain in acid practice, although in the one case the product finds a resting-place, while in the other it is instantly destroyed.
During the elimination of carbon, a small quantity of phosphorus is burned and held by the slag, but for practical purposes the percentage at the drop of the carbon flame is equal to the initial content. From that time the phosphorus seizes the oxygen in the same way as the silicon and carbon had done before, and the iron is perfectly protected, the phosphoric acid immediately uniting with the lime. It might be supposed that any other base like oxide of iron would serve to hold the phosphorus, but phosphate of iron is easily reduced by carbon, and in other respects iron oxide is inferior to the oxide of calcium which gives a stable compound.
SEC. Vile.—Amount of lime required.—The amount of lime needed will depend upon three conditions, viz.:           <
(1)   The amount of silicon in the pig.
(2)   The amount of phosphorus in the pig.
(3)   The quality of the lime.
If the charge is 15,000 pounds, containing 0.50 per cent, silicon, it will produce 160 pounds of silica; and if the final slag must contain 6.0 per cent, silica, then the slag must weigh 2670 pounds; and if it must have 50 per cent. CaO, then 1335 pounds of unsatisfied CaO must be added. The qualification is inserted that it must be "unsatisfied/5 for each pound of silica in the lime detracts from its efficacy. Thus, if the lime contains 2 per cent. Si02, there will be 2 pounds of silica in every 100 pounds of addition, and if this is to be made into a slag containing 6 per cent, of Su)2 and 50.0 per cent, of CaO, then 8 pounds of CaO is useless, since vt will